1. Field of the Invention
The present invention relates to a balancing structure. In particular, the present invention relates to a balancing structure for a rotor of a motor.
2. Description of Related Art
A wide variety of structures for maintaining rotational balance for a rotor of a motor have been proposed. One of these structures comprises a fixed portion, a rotor, an attracting portion, and a magnetically conductive portion. The fixed portion includes a base, an axial tube fixed on the base, a bearing received in the axial tube, a stator, and a circuit board. The stator includes a coil with axial winding or radial winding and a plurality of pole plates (or pole arms). The rotor includes a shaft rotatably extending through the bearing and an annular magnet surrounding the pole faces of the pole plates. The attracting portion is provided on a bottom of the rotor or a top end of the axial tube. Alternatively, the attracting portion is provided by the annular magnet or the alternating magnetic fields created by the pole plates of the stator. The magnetically conductive portion is made of a magnetically conductive material and may be comprised of a disc with two arcuate edges, a casing fixed in a rotor housing of the rotor, an annular plate, a plurality of arcuate plates, or a rotor housing of a spindle motor. The magnetically conductive portion may be provided on the circuit board, an inner periphery of the rotor, or the bottom of the rotor, and associated with the attracting portion. Such a structure is disclosed in, e.g., Taiwan Utility Model Publication Nos. 383818, 422365, 428838, 590330, and M241969, U.S. Pat. Nos. 6,097,120; 6,483,209; 6,700,241; and 6,727,626, and U.S. Patent Publication No. 2005/0006962.
When the motor turns, alternating magnetic fields are created by the pole faces of the magnetic pole plates (or pole arms). The magnetically conductive portion is attracted by the attracting portion during rotation of the rotor, thereby maintaining rotational balance of the rotor, avoiding disengagement of the rotor shaft from the stator, reducing rotational friction of the bearing, and prolonging the life of the motor.
In the motors disclosed in U.S. Pat. Nos. 6,483,209 and 6,727,626, each pole face of the magnetic pole plates includes a strong magnetic section and a weak magnetic section. At the moment of starting the motor by energizing the winding, the strong magnetic section on each pole face creates stronger alternating magnetic fields whereas the weak magnetic section on each pole face creates weaker alternating magnetic fields, resulting in uneven alternating magnetic fields for driving the rotor to turn.
However, problems still exist in the above-mentioned rotational balancing structures for easy-to-start motors. For example, the metal plate consisting of the magnetically conductive portion is made as large as possible so as to provide a large area facing the annular magnet of the rotor. This may result in excessive attracting force between the magnetically conductive portion and the attracting portion for balancing the rotor. Further, the magnetically conductive portion often extends across the alternating magnetic fields created by the pole faces of the stator, particularly across the stronger magnetic sections, which adversely affects the alternating magnetic fields and lowers the rotational efficiency of the rotor. Further, although provision of the strong magnetic section and the weak magnetic section on each pole face is advantageous to easy starting, the local uneven energizing resulting from the strong magnetic sections and the weak magnetic sections results in vibration and shaking at the moment of starting the rotor. The rotational wear to the bearing could not be reduced and the life of the motor could not be prolonged. Further, the actual effect of the rotational balancing structure was not satisfactory as expected.